1. Field of the Invention
The present invention relates to a method of fabricating integrated semiconductor devices comprising a chemoresistive gas microsensor.
2. Discussion of the Related Art
As is known in the art, chemical sensors detect the presence of a gas by virtue of a chemical reaction between the molecules of a gas and a sensitive film. The chemical reaction depends to a large extent on operating temperature, which affects the adsorption, desorption and diffusion of the gas in the film. Therefore operating temperature is an important factor in optimizing the performance of the sensor, particularly in terms of sensitivity, selectivity and response time. Thus, sensors are equipped with temperature regulating and control means that allow optimization of operation.
In recent times, integrated chemoresistive gas microsensors have been fabricated using microelectronics technology. Such sensors present numerous advantages: low fabrication cost; low in-service energy consumption; rapid response time; and integration with the temperature control and output signal processing circuit.
Integrated gas microsensors which are now being marketed feature chemoresistive tin oxide diaphragms deposited on a wafer of bulk-micromachined semiconductor material, and detect the presence of gas as a change in the resistance of the film caused by a chemical reaction, on the surface of the diaphragms between the oxygen of the diaphragm and the gas.
To function properly, the sensors must be maintained at a temperature of about 400.degree. C. Therefore, they are provided with heating elements, and must be thermally isolated from the rest of the chip integrating the signal control and processing circuit.
Various techniques are known for isolating the sensitive portion from the rest of the chip. The traditional technique is bulk micromachining, which consists of forming the sensitive portion on or in a dielectric layer deposited on a massive silicon wafer, and in removing a portion of the silicon from the rear of the wafer by plasma or wet etching. The dielectric layer performs the dual function of mechanically supporting the sensor and thermally isolating it from the silicon wafer. Using this technique, prototypes have been formed wherein part of the silicon is removed from the sensor area and only part of the thickness of the wafer is etched, whereas in other prototypes, all the silicon is removed at the sensor area (etching extending up to the dielectric layer supporting the sensor element). Details of the latter solution are to be found, for example, in the article entitled "Basic micro-module for chemical sensors with on-chip heater and buried sensor structure" by D. Mutschall, C. Scheibe and E. Obermeier.
Bulk micromachining, however, requires front-rear processing and such particular handling of the wafers as to be incompatible with current integrated circuit fabrication methods.
Another technique is front micromachining, whereby the massive silicon wafer or a sacrificial layer is etched from the front, and a dielectric layer mechanically supports and thermally isolates the sensor element. Details of this technique, relative to the fabrication of a different type of sensor, are to be found in the article entitled "A high-sensitivity CMOS gas flow sensor based on an N-poly/P-poly thermopile" by D. Moser and H. Baltes, DSC-Vol 40, Micromechanical Systems, ASME, 1992. A general review of bulk and front micromachining technology is found in the article entitled "Micromachining and ASIC technology" by Axel M. Stoffel, Microelectronics Journal, 25 (1994), p. 145-156.
Forming suspended structures using this technique, however, involves etching steps which are incompatible with current microelectronics fabrication processes, so that the sensors and relative control and processing circuits cannot be formed on one chip.
For sensors of a different type, dedicated SOI (Silicon-on-Insulator) substrates have been proposed, wherein the starting wafer comprises a Silicon-Silicon Oxide-Silicon stack with the oxide selectively removed at the sensor area to form an air gap. The trenches formed in the front of the wafer after contacting the air gap provide thermal isolation for the sensor. Details of this technique, relative to a shear stress sensor, are to be found, for example, in the article entitled "A Microfabricated Floating-Element Shear Stress Sensor Using Wafer-Bonding Technology" by J. Sliajii, Kay-Yip Ng and M. A. Schmidt, Journal of Microelectromechianical Systems, Vol. 1, N. 2, Jun. 1992, p. 89-94. The bonding technique used (excluding formation of the air gap) is also described in the article entitled "Silicon-on-Insulator Wafer Bonding-Wafer Thinning Technological Evaluations" by J. Hausman, G. A. Spierings, U. K. P. Bierman and J. A. Pals, Japanese Journal of Applied Physics, Vol. 28, N. 8, Aug. 1989, p. 1426-1443.